Age of acquisition effects on sign language development and brain processing Age of first-language acquisition, L1 AoA, has enduring effects on adult language abilities and brain language processing. Using deaf signers' acquisition of ASL as a model for the critical period for language, this project investigates three questions. How does L1 AoA affect the development of simple compared with complex and inter-sentential structure? Does the brain process language differently when language development begins at older as compared to younger ages? Are there links between language development and brain language processing? The language and neuroimaing studies employ a retrospective, cross-sectional design. Comprehension and production of American Sign Language, ASL, is investigated with sentence-to-picture matching tasks with accuracy and reaction time as dependent measures, and detailed linguistic analyses of picture descriptions given in ASL. Brain language processing is investigated with aMEG and fMRI using match-mismatch designs with pictures and signs. L1 AoA is controlled by testing two unique groups: one is deaf adoptees raised by ASL signing families; the other is deaf immigrants who learned ASL in school after immigrating to the USA with their hearing, non-signing families. Individuals in both groups acquired language in either early or late childhood. Comparing the language and brain language processing of the two groups helps determine if the amount of input during language development mitgates L1 AoA effects on language and brain processing. One control group is deaf adults who learned ASL from infancy; their results show language and brain processing patterns associated with an onset of language acquisition in infancy. A second control group is L2 learners of ASL; their results show what language brain processing patterns look like when a L2 is learned in later life. The language hypotheses are that simple sentence structures can be acquired at any age but that complex and inter-sentential structure cannot, both in comprehension and production. The brain processing hypotheses are that an early onset of language acquisition is necessary for the classic language network in the left hemisphere to develop fully. When the onset of language acquisition is late, posterior regions responsible for visual and spatial processing predominate language processing. Level of language skill is predicted to relate to these two types of brain language processing. The results will provide information crucial for clinical and educational services for the deaf population as well as for theory building about the relation of language to brain development.